Certain phosphoroamidothioates and phosphoroamidodithioates (collectively referred to as "phosphoroamidothioates" for convenience) are known in the art as having excellent insecticidal activity against a variety of insects and in a variety of environments. A particularly important commercial insecticide within this class of compounds is the insecticide acephate (generic name) or Orthene.RTM. (trade name), which can be systemically taken up by a plant so that insects which feed and/or live on the plant are killed, in addition to those insects which directly ingest or are contacted by the insecticide. Acephate and related compounds are described in U.S. Pat. Nos. 3,716,600, 3,845,172 and 3,914,417, which disclose that in addition to their insecticidal properties, the compounds possess very low mammalian toxicity. Orthene.RTM. is commercially produced as a technical grade chemical of about 97 to 99.5% purity, which is often referred to as acephate technical or acephate TG (technical grade).
Acephate technical is commercially available as a powder, which has a tendency to clump or agglomerate. In the past, acephate technical has been applied to crops as a dust (for example, after exposure to moisture via rain, dew or irrigation), or in spray form as a water solution spray. Dusts are undesirable because of airborne contamination and handling difficulties, while liquid spray formulations involve solvent and packaging expenses, and container disposal requirements that detract from commercial desirability.
Accordingly, in recent years, attention has turned to a pellet form of application for acephate and its related insecticidal compounds. Pellets have the practical advantages of eliminating dust problems and reducing offensive odors in comparison to powder forms because of a reduced surface area to weight ratio. However, problems have been encountered in these pelletization efforts and to date, high-strength acephate pellets have not been commercialized, most likely due to processing difficulties.
Phosphoroamidothioate-containing pellets have been proposed in U.S. Pat. Nos. 5,075,058, 5,100,667 and 5,464,623, each of which is assigned to Chevron Research and Technology Company. Each of these three patents recognizes the difficulties in pelletizing acephate technical, and proposes certain types of pellets made, for example, by extrusion. For example, the '058 Chevron patent states in the paragraph bridging columns 2-3 that attempts to manufacture acephate technical pellets from acephate technical powder "have heretofore been unsuccessful". Similar statements are made in each of the '667 and '623 Chevron patents. The present inventors have conducted considerable experimentation in the area of producing high-strength acephate pellets, and have confirmed the manufacturing difficulties which the Chevron inventors apparently experienced. Furthermore, the pellets and methods proposed for making them as discussed in the three Chevron patents leave considerable room for improvement. In this regard, while these patents generally describe extrusion of solid pellets, few details of the actual extrusion process are set forth. Moreover, the Chevron patents generally propose the addition of costly surfactants (see '667 patent), the combination of the phosphoroamidothioate with a second active ingredient (see '058 patent), and the creation of a mixture of the active ingredient with a solvent in an amount of from 3-25% by weight before extrusion.
To satisfy the need in the art for a low cost, chemically stable, insecticidally active phosphoroamidothioate granular formulation, one of the present inventors developed an insecticidally active composition comprising particles prepared by the compaction of an admixture comprising ammonium sulfate and at least one insecticidally active phosphoroamidothioate. This composition and methods for its manufacture and use are described in U.S. Pat. Nos. 5,298,501, 5,352,674, and 5,369,100. As set forth in these patents, it was found that the use of ammonium sulfate in particular leads to a compacted composition which provides a high degree of chemical stability.
U.S. Pat. No. 5,443,764 issued Aug. 22, 1995, and related U.S. Pat. No. 5,622,658 issued Apr. 22, 1997, each assigned to ICI Australia Operations Proprietary Ltd., disclose a process for the preparation of water-dispersible granules. The process comprises mixing the desired ingredients of the granules to form an extrudable composition, extruding the mixture and rolling the extrusions to break them down into granules. In contrast to the pellets of the present invention, the granules disclosed in these ICI patents are designed to have rapid dispersion and superior suspensability in water. The granules of the ICI patents comprise an active agricultural chemical (including, e.g., insecticides) and normally have a surfactant component and/or a binding agent. The particular class of insecticidal compounds included in the pellets of the present invention are not disclosed in the ICI patents, although a wide variety of insecticidal active ingredients are stated to be useful.
Thus, there is a need in the art for chemically stable, high strength, insecticidally active phosphoroamidothioate pellets which are useful from a practical standpoint, as well as for a low cost, practical manufacturing technique which can be practiced on a commercial scale without requiring expensive additives or solvents.